1. Field of the Invention
The present invention relates to a chain drive mechanism which is capable of transmitting a rotary or linear movement to an associated mechanism via its chain.
2. Prior Arts
Such a rotary or linear movement transmission comprises two sprocket wheels each having a plurality of teeth formed on its circumference and being separated a given distance, and a length of chain passing round the opposite sprocket wheels.
The profile of the sprocket tooth is defined in many national and international standards. These conventional design use a combination of several circular arcs and straight lines for the profile. For example, FIG. 8 shows a typical conventional design of the tooth profile, where the profile of the tooth C1 of the sprocket wheel is determined as the combination of two circular arcs R1 and R2. Sometimes a straight line is inserted between the two arcs (not shown). Each radius of the two arcs is bigger than the radius R4 of the roller C2. When in transmission, as shown in FIG. 9, there exist gaps D1 and D2 between the roller and sprocket teeth. This gap or backlash causes loss of movement when the transmission is made in reverse direction. This non-theoretical tooth profile causes uncertainty of power transmission, and thus produces noise and vibration.
It is known that the theoretical contact between a circle and a line makes the involute curve as a contour of a point on the line. It is seemed, from this theory, that it is better to use an involute curve as the roller center path for the sprocket tooth. However, it leads to xe2x80x9cundercutxe2x80x9d of the profile, as described later, and precise transmission cannot be expected.
One object of the present invention is to provide a chain drive mechanism which is capable of transmitting a rotary movement with precision, causing neither noise nor vibration, and is guaranteed to be free of backlash in reversing the motion.
Another object of the present invention is to provide a method of making a sprocket wheel appropriate for use in such a backlash-free chain drive mechanism.
With our invention, an adequate amount of offset is given between the roller chain pitch line and contact pitch line. By this a theoretically complete contact between rollers and teeth is made throughout the movement without any backlash.
A chain drive mechanism according to the present invention comprises: two sprocket wheels each having a plurality of teeth formed on its circumference, the sprocket wheels being separated in a given distance apart from each other; and a closed loop of chain having means for engaging with the teeth of each sprocket wheel, passing round the opposite sprocket wheels; the means for engaging with the teeth of each sprocket wheel comprising a plurality of rollers arranged at regular intervals, the rollers being so supported in the closed loop of chain as to rotate about their shaft pins, and the profile of each tooth being in conformity with the envelope curve traced by each roller when the roller moves with its center following an involute-trochoid curve, which is determined from the linear movement of the roller and the rotating movement of the sprocket wheel.
The involute-trochoid curve is a theoretical curve determined by displacing the roller center pitch line a certain distance apart outside from the contact pitch line, and by rotating the contact pitch line on the base pitch circle of the sprocket wheel with no slip. Then the roller center point traces the involute-trochoid curve. The certain distance being so determined that the roller pitch length may be equal to the incremental arc length, which is determined by dividing the circumference of the base circle by the number of teeth.
The profile of the tooth is determined by taking account both the linear movement of the roller and the circular arc movement of the sprocket wheel. And therefore, the so determined profile of the tooth assures that each roller precisely contacts to the profile all the time, smoothly moving from tooth to tooth without causing noise and vibration, and that a rotary movement is transmitted with high precision.
The chain drive mechanism may comprise some means for applying pressure strong enough to stretch the closed loop of chain tight between the opposite sprocket wheels. This has the effect of keeping the roller-to-tooth contact condition unchanged all the time. The pressure applying means may include means for increasing the center-to-center distance between the opposite sprocket wheels.
A method of manufacturing the sprocket wheel comprises the step of rotating and moving a cutting tool having the same diameter as the roller with its center following the involute-trochoid curve, which is determined from the linear movement of the roller and the circular arc movement of the sprocket wheel.
Prior to description of one preferred embodiment of the present invention, the principle of the present invention according to which the tooth profile is determined is described below.
First, assuming that a roller contacts on a theoretical involute-derived tooth as shown in FIG. 2, and that the roller stays on Point C on the tooth with its center P on the involute curve, the radius of curvature of the profile xcfx81C, at Point C is given by:
xcfx81C=xcfx81xe2x88x92rC
where xcfx81 stands for the radius of curvature of the roller center path at Point P, and is equal to rxcex8; rC stands for the radius of the roller; xcex8 stands for the angle of rotation of the roller pitch line rotated from the starting point O and r stands for the base circle radius.
In this case, undercut appears on the tooth profile where rC is larger than xcfx81 (rC greater than xcfx81), disturbing normal transmission of movement. The radius of curvature xcfx81 at the start point of the involute curve is zero, and therefore, an undercut will inevitably appear at the tooth bottom even if a roller of minimum size is used. Appearance of such undercuts in the tooth bottom is the cause for lowering the precision with which the roller-and-sprocket drive works.
The inventors discovered that this defect can be eliminated by so designing the tooth profile that the roller pitch line of the chain may be offset a certain distance above the contact pitch line (see FIG. 4), thereby permitting the center of the rotating roller to follow a generic trochoid curve (herein called xe2x80x9cinvolute-trochoidxe2x80x9d curve). Then, the minimum radius of convex curvature xcfx81min of the involute-trochoid curve cannot be zero, so that no undercut may appear at the tooth bottom. The offset amount by which the roller pitch line is put apart from the contact pitch line to remove the undercuts which would otherwise appear is so determined that the minimum radius of convex curvature xcfx81min may be equal to or larger than the radius of the roller (xcfx81minxe2x89xa7rC).
This requirement can be realized by moving the roller with its center following an involute-trochoid curve. Stated otherwise, the tooth needs to be so profiled that the center of the roller may be allowed to follow the involute-trochoid curve. The required tooth profile can be determined as follows.
The roller pitch line is put a certain distance apart from the contact pitch line, and the contact pitch line is rotated round the base circle with no slip to allow a fixed point on the roller pitch line to trace an involute-trochoid curve, as seen in FIG. 5a. A roller J8 is rotated with its center following the involute-trochoid curve, and then points which are apart from the involute-trochoid curve by a distance equal to the radius of the roller J8 are traced to draw the envelope curve of the rotating roller, which gives the required profile of the tooth, as seen in FIG. 5b. 
To satisfy the tight chain-winding requirement simultaneously with disappearance of undercuts a certain distance is so determined that the roller pitch may be equal to the incremental arc length, which is determined by dividing the circumference of the base circle by the number of teeth, as described later in detail.
In making the profile of the sprocket as described above, a cutting tool having the same diameter as the roller is rotated and moved with its center following the involute-trochoid curve.
The rollers are so connected to the chain that they may rotate about their shafts, thereby permitting them to rotatably contact the teeth of the sprocket, and therefore the friction between the roller and the tooth can be reduced to minimum, and accordingly the life of the roller-and-sprocket transmission can be extended.
The roller-and-sprocket drive according to the present invention permits two rollers to keep contact from both sides of a particular tooth throughout the whole engagement. Thus, the gapless, smooth engagement can be established both at the linear-to-circular arc transition and at the circular arc-to-linear transition. This is impossible in a conventional roller-and-sprocket drive.
By the gapless engagement attained at all part of the sprocket, the roller-and-sprocket drive according to the present invention is guaranteed to be free of backlash when reversed in motion. The roller-and-sprocket drive, therefore, can be used in a servo control system for precise positioning.
Also, it can make linear movement on the basis of the theoretical constant-speed motion, permitting transmission of power without accompanying noise and vibration.